1. Field of the Invention
The present invention relates to diffusion imaging using a magnetic resonance imaging device and, more particularly, to a technique for modifying the RARE sequence to eliminate artifacts which relate to the strong sensitivity of multiple spin echo sequences to the phase of the prepared magnetization so that RARE may be used for diffusion and T2* imaging.
2. Description of the Prior Art
Recent advancements in scanner hardware and pulse sequence design have made possible single excitation images of excellent quality based on multiple spin echo sequences (MSES) such as RARE and GRASE. These sequences are an attractive alternative to echoplanar imaging because they suffer much less from chemical shift artifact and distortion than echoplanar. Echoplanar imaging is frequently used for forms of functional imaging in which a preparation sensitive to physiology is applied prior to the echoplanar readout. Such an approach can, in principle, also be used to prepare MSES.
Those preparation schemes that affect longitudinal magnetization, such as magnetization transfer saturation, arterial spin tagging, inversion recovery and saturation recovery, do not affect image quality and consequently work well with MSES. However, preparation schemes that alter transverse magnetization such as T2, diffusion, and T2* preparation can result in severely degraded images because they introduce unknown phase shifts in the transverse magnetization due to motion, chemical shift or magnetic field inhomogeneity. Most MSES rely on the amplitude stability of the Carr-Purcell-Meiboom-Gill (CPMG) sequence which depends upon the Meiboom-Gill (MG) phase condition so any changes in the phase of the transverse magnetization caused by preparation will result in rapid attenuation and modulation of the echo amplitudes causing signal loss and blurring in the images.
For T2 preparation, phase errors will only occur if the timing of the sequence is inaccurate or subject motion in the presence of the relatively weak crusher gradients causes phase errors. Accurate timing and moderately cooperative subjects will probably be sufficient to obtain good quality T2 prepared images. T2 weighting can also be obtained by appropriate phase encode order.
On the other hand, T2* preparation will only be successful if all spins have the same chemical shift and the shim is outstanding. Though T2* weighted images will always exhibit signal loss in voxels where the static magnetic field gradient is very large, T2* prepared MSES images will also show severe signal loss or blurring in regions where the static magnetic field offset is large. This extra sensitivity to shim makes conventional T2* prepared MSES imaging unattractive.
Phase errors due to motion in the presence of large magnetic field gradients are the reason for the extreme motion sensitivity of multi-shot diffusion imaging. Single-shot echoplanar imaging is frequently used for diffusion imaging to avoid these motion induced errors. However, if a phase sensitive MSES is employed for diffusion imaging, severe signal loss and attenuation will occur.
The phase sensitivity of the CPMG sequence is clearly undesirable for these applications, so ways to avoid this sensitivity must be sought. The simplest approach is to ensure that the refocusing flip angle is exactly 180xc2x0. A MSES with exactly 180xc2x0 refocusing pulses is completely insensitive to phase but very small deviations from 180xc2x0 are sufficient to introduce artifacts. These artifacts are caused by the presence of multiple stimulated and spin echo pathways to produce signal contribution at the echo time. For magnetization satisfying the MG phase condition, these pathways add constructively and eventually achieve a temporary steady state echo amplitude. If magnetization is 90xc2x0 from the MG phase, then the pathways interact destructively causing signal amplitude decay and oscillation. In most practical applications, including multi-slice imaging where the flip angle is not uniform across the slice, the flip angle cannot be made close enough to 180xc2x0 to eliminate signals from these other pathways. In long echo train applications, such as single shot imaging, it is also desirable to lower the refocusing flip angle to minimize the power deposition in the subject. The favorable properties of reduced flip angle CPMG sequences have been described by Alsop in an article entitled xe2x80x9cThe Sensitivity of Low Flip Angle RARE Imaging, xe2x80x9d Magn. Reson. Med., Vol 37, pp. 176-184 (1997) and by J. Hennig in an article entitled xe2x80x9cMultiecho Imaging Sequences with Low Refocusing Flip Angles,xe2x80x9d J. Magn. Reson., Vol. 78, pp. 397-407 (1988).
C. S. Poon et al. in an article entitled xe2x80x9cPractical T2 Quantitation for Clinical Applications,xe2x80x9d JMRI, Vol. 2, pp. 541-553 (1992) proposed crusher gradient schemes that can eliminate all but the primary refocused component. Unfortunately, these schemes require a large crusher amplitude that increases linearly with echo number. The added time required to apply the crusher gradients generally becomes unacceptable after only a handful of echoes. Several investigators have reported such sequences using crusher amplitudes that are too weak to fully dephase an individual voxel. These sequences employed non-selective refocusing pulses very close to 180xc2x0 so the unwanted signal components are very weak. The quality of images obtained with slice selective or reduced flip angle refocusing pulses and these weaker crusher gradients would have to be evaluated. This spoiling approach has been employed to acquire. single-shot GRASE diffusion images. Because very few spin echoes and many gradient echoes were employed, the sensitivity to chemical shift and susceptibility artifacts was comparable to echoplanar. The highest quality GRASE images tend to employ many more radio frequency (RF) pulses and only a few gradient echoes. Spoiling of the CPMG sequence in this way will also cause the echo amplitudes to decrease rapidly with echo number if the refocusing pulse is reduced significantly from 180 degrees so reduction of the refocusing flip angle to lower power deposition is not possible.
A number of modifications to the CPMG sequence have been proposed to reduce errors in T2 quantification or artifacts in multiple echo images by modulating the phase of the refocusing pulses. Some of these sequences can be interpreted as employing composite 180xc2x0 pulses which are more insensitive to RF amplitude errors. Though these sequences work well for a few echoes when the flip angle is already near 180xc2x0, they begin to fail if the amplitude of the RF is reduced more significantly. They also usually increase the echo spacing and make single shot imaging more difficult. A two excitation method for producing phase insensitive images has also been proposed. Since the source of the phase uncertainty in diffusion imaging, motion, is not reproducible from excitation to excitation, the two shot method is not applicable.
An alternate approach to eliminating phase sensitivity of MSES sequences by crushing only some of the many stimulated and spin echo pathways has been presented by Norris et al. This approach was designed to overcome hardware limitations that precluded the precise timing and control necessary to achieve the MG phase condition. Modern clinical hardware can now readily achieve the MG condition because RARE has become an essential clinical tool. For the special applications of diffusion and T2* prepared RARE imaging, however, this approach is still very important. A limitation of the method is the large number of echoes which must be discarded before the signal is sufficiently stable to begin phase encoding.
A refinement of the Norris et al. method is desired that permits acquisition of data from the very first echo for acquisition of diffusion images. The present invention has been developed to meet this need in the art.
The present invention relates to a modification of Multiple Spin Echo Sequences (MSES), such as RARE, Fast Spin Echo, and GRASE, which makes image quality unaffected by the initial phase of the spins. This modification makes it possible to acquire images with diffusion or T2* contrast with a MSES. Diffusion imaging has been shown to be highly accurate at the detection of acute stroke and T2* imaging can be useful for detection of hemorrhage and the imaging of certain tissues after the administration of contrast agents. The images resulting from this modified MSES sequence are free from the spatial distortion artifacts which plague the single excitation echoplanar images most widely used for diffusion imaging today.
In accordance with the invention, additional dephasing gradients and RF pulses are used to eliminate phase errors that are normally associated with diffusion and T2* single-shot MSES imaging. Following standard diffusion or T2* sequences which leave the spins pointing along the plane perpendicular to the scanner magnetic field, a MSES imaging sequence is performed with several modifications which together comprise the invention. In particular, the invention comprises the steps of:
applying a first dephasing magnetic field gradient to the sample to generate a magnetization;
applying a first radio frequency pulse to the sample which rotates the magnetization by approximately 90xc2x0, the first radio frequency pulse having a phase which is the same as a phase of refocusing pulses of a spin echo sequence to be applied to the sample;
applying at least one spin echo sequence whereby a first refocusing pulse of each spin echo sequence occurs at a time after said first radio frequency pulse which is half a time duration between successive refocusing pulses of the spin echo sequence, each spin echo sequence including:
a second dephasing magnetic field gradient applied to the sample along the same direction as the first dephasing gradient,
a data acquisition period for acquiring data representative of the sample, and
a second radio frequency pulse having a sign opposite the first radio frequency pulse; and
generating an image of the sample from the data acquired during each data acquisition period.